In surgical procedures, it is often desirable to use a cannula to penetrate an organ or vessel under fluid pressure. For example, a number of ophthalmic surgical procedures involve penetrating the chambers of the eye with surgical instruments. The anterior chamber of the eye contains a physiologic fluid and is located between the cornea and the lens of the eye. The vitreous chamber between the lens and the retina is filled with vitreous humour, a transparent gel-like substance. The configuration of the ocular chambers is maintained by fluid pressure.
When surgery is performed, the penetration of a surgical instrument and, to a greater extent, the removal of fragments of ocular tissue, such as cataractous tissue or other diseased tissue, by aspiration results in loss of fluid and pressure in the respective ocular chamber. Furthermore, it will be appreciated by those skilled in the art that, under surgical conditions, there may be fluctuations in pressure in the ocular chambers due to surgical manipulations, such as starting or enlarging an incision, inserting instruments into the incision, pulling on ocular muscles or tightening of sutures.
Loss of fluid and pressure in the ocular chambers during ophthalmic surgical procedures can cause at least partial collapse of the chamber making it difficult to manipulate an instrument within the eye without endangering vulnerable structures such as the corneal endothelium, the lens capsule and the iris. Clearly, it is desirable to maintain the integrity and internal pressure of the ocular chambers during and after surgery.
One accepted technique for maintaining pressure in the ocular chambers is to introduce fluid, such as air, saline solution or other liquid or gas, into the anterior chamber and/or the vitreous chamber either during the ophthalmic surgical procedure or at the time of closure of the incision(s). The infusion of fluid helps to maintain a positive pressure and the normal configuration of the ocular chamber during surgery. Furthermore, control of intraocular pressure during surgery helps to minimize danger to the eye during and after surgery. Positive pressure stretches open the pupil permitting better visualization of posterior structures. It also ensures that any flow across a wound is outward, not inward, reducing the risk of entry of foreign matter or bacteria.
Fluid is typically introduced to the ocular chambers via a so-called chamber maintainer cannula inserted through a surgical stab wound or other incision. The chamber maintainer cannula is connected via silicone elastomer tubing or other suitable tubing to a container holding infusion fluid. The flow of infusion fluid through the chamber maintainer cannula is controlled by the surgeon or an assistant in response to a surgeon's instructions, for example by operation of a foot pedal, by fingertip control or by altering the height of an infusion bottle.
In retinal surgery, the chamber maintainer cannula is inserted into the vitreous chamber through the sclera, which is a fibrous tissue intermixed with fine elastic fibres. For anterior segment eye surgery, the cannula is inserted directly through the peripheral cornea or via the anterior sciera to the peripheral cornea into the anterior chamber. The cornea, in contrast with the sclera, has a thick central layer which is fibrous, tough and unyielding.
One known chamber maintainer cannula is a smooth-walled uniform diameter shaft which may have an angled tip to facilitate entry through an incision made in the sclera and/or the peripheral cornea. The cannula is held in position by sutures. However, the sutures require an additional surgical procedure. Moreover, the patient is further traumatized by the suturing. One attempt to overcome the need for sutures is to provide a cannula which has an etched surface. However, the present inventor and other practitioners have found that the etched cannula does not perform satisfactorily and, especially upon introduction of fluid, still requires the use of sutures to hold the cannula in position. Furthermore, even with the use of sutures to secure the smooth-walled and etched cannulae, there is some back-flow of fluid from the pressurized ocular chambers. This problem is acknowledged in "Maintenance of the Anterior Chamber in Intraocular Surgery: Instruments and Techniques" (Page, R. N. et al Trans Pacific Coast Oto-Ophthalmolopical Society 161-173; 1968).
U.S. Pat. No. 4,331,130 (Lewicky, May 25, 1982) relates to a device for preventing collapse of the anterior chamber during ophthalmic procedures comprising a pneumatic pump, a liquid pump and a fluid flow connection for connecting the pumps to an infusion terminal. The infusion terminal has a shank with a tapered rounded head and an axially extending central fluid-flow passageway for the introduction of fluids into the eye. The shank has helical external screw threads or annular detents, rings or circular ribs which extend outwardly therefrom. Lewicky teaches that the maximum outside diameter of the threads or detents, as well as the shank, should be smaller than the diameter of the surgical opening. Specifically, the opening in the eye should be one to two gauge sizes larger than the outside diameter of the threads or rings.
The method for preventing collapse of the anterior chamber of the eye during ophthalmic surgical procedures is described in U.S. Pat. No. 4,340,037 (Lewicky, Jul. 20, 1982). According to the method of Lewicky, a partial-thickness groove is made in the sclera or external coat of the eye and two sutures are pre-placed in this groove. A 22-gauge disposable needle is used to create a full-thickness corneal tract to the anterior chamber. The needle is withdrawn and a 23-gauge infusion terminal is inserted through the corneal tract. The infusion terminal is connected by a fluid line to a pump for infusion of a balanced salt solution by control of foot pedals by the surgeon.
The present inventor has personal experience with the Lewicky infusion terminal and has found that the terminal stays in position well and reduces the back-flow around the terminal, but only if the terminal is tightly wedged into the entry wound. However, the present inventor has also found the terminal awkward to insert unless the stab wound is large. The size of the stab wound or incision is critical for application of Lewicky's terminal since the threads or rings and the shank are of a uniform outside diameter along the length of the terminal and because, as previously indicated, the cornea is a rigid structure. It will be appreciated by those skilled in the art that, if the stab wound is too small, insertion of the terminal is very difficult. If the stab wound if too large, the fit between the terminal and the wound is too loose to retain the terminal snugly. A loose fit between the wound and the terminal can result in back-flow of fluid out of the wound, particularly during infusion of fluids into the ocular chambers. Moreover, the terminal can be dislodged from the wound, in which case the surgical operation must be either interrupted or terminated.
In spite of the numerous advantages of using a chamber maintainer, as discussed above, few eye surgeons working in the anterior chamber routinely use one, relying instead on disposable viscoelastic agents to form the anterior chamber and to protect intraocular structures. However, these are expensive (adding many millions of dollars annually to the cost of cataract surgery alone in the United States) and they can block aspiration of anterior chamber fluid during ultrasonic fragmentation of the nucleus in cataract surgery resulting in severe wound burns. The viscoelastic agents are also very difficult to completely extract from the eye and can raise the post-operative intraocular pressure resulting in a transient but nevertheless potentially vision-damaging form of glaucoma.
The main reasons why surgeons eschew the use of a chamber maintainer are inconvenience and difficulty with their insertion and lack of confidence in their retention.
It is an object of the present invention to provide a cannula which will not become unintentionally dislodged once in position.
It is another object of the present invention to provide a cannula which is easily inserted and removed.
It is a further object of the present invention to eliminate the requirement for sutures to hold the cannula in position.